The present invention relates to fabrication of microelectronic circuits and more particularly to a structure and method for programmably adjusting a matched impedance to reduce signal reflection at an interface to a transmission line.
The continuous increase in the speed and complexity of digital communication systems leads to a strong demand for improved methods of data transmission in these systems. One challenge is the presence of reflection along transmission lines that carry data between components of the system. A reflection is a return of energy (i.e. electromagnetic energy) caused by an impedance mismatch occurring along a transmission line. Stated another way, any pulse or signal propagating along a transmission line will reflect at any point where the transmission line becomes different.
Many things can lead to impedance mismatch and, therefore, reflection along a transmission line. For example, variations in manufacturing process can cause elements of a microelectronic package that should be identical to vary from one to another, resulting in certain components having higher or lower impedance than desired. Wiring and interconnections in the microelectronic package and even printed wiring boards and passive components are all subject to variations in the manufacturing process and changes due to temperature, thus contributing to impedance mismatch and jitter noise.
No matter what the exact cause of the reflection is, it is important to keep reflection along transmission lines to a minimum. Depending on the strength of the transmitted signal and the ratio of the transmitted signal to the reflected signal, the transmitted signal and the reflected signal may either cancel each other out or otherwise interfere. The conventional solution to address reflection is to terminate the transmission line in an impedance matched to the characteristic impedance of the transmission line. Such matched impedance is ordinarily provided in form of a resistor, known as a termination resistor.
Ideally, the circuitry within an integrated circuit (hereinafter “IC” or “chip”) which is responsible for transmitting or receiving off the chip must be designed in a way that addresses reflection and impedance mismatch. Although these concerns are addressed somewhat by use of on-board termination resistors to terminate transmission lines, there are problems that hinder achievement of this goal. Most IC's are designed so that the IC communicates with off-chip devices through metallization patterns on the exterior of the chip known as signal pads. For communication between the chip and off-chip circuitry, input output (I/O) interface circuits transmit and receive data through the signal pads. The signal pads of like and/or different chips are connected together by transmission lines on one or more printed wiring boards and/or wires or cables within or between communication systems, thereby allowing communications between the various chips of a system and/or between a plurality of systems.
Difficulties in chip design arise when the length of on-chip wiring between input output (I/O) devices and the bonding pads causes such on-chip wiring to act more like a transmission line, affecting signal integrity. The impedance of the on-chip wiring, as well as its length in such instances becomes considerable due to layout and pad limitation. The problem is further aggravated by parasitic effects, that is the of unwanted resistive, capacitive and/or inductive elements of the chip or package. This is especially true when a large size device is added to protect the chip from being damaged by an unexpected high current surge, such as for electrostatic discharge protection. In such instances, a simple termination resistor will not adequately provide a matching impedance to the characteristic impedance ZO, of the transmission line because the large size ESD device adds a reactive (inductive or capacitive) term that cannot be matched with a resistor alone.
Many approaches have been used to reduce reflection that occurs at interfaces to transmission lines. In one approach, a transmission line is terminated in an impedance that is matched to that of the transmission line. For example, for a pair of transmission lines on which differential signals are transmitted, a pair of termination resistors, each having a value of “R” corresponding to the characteristic impedance “ZO” of one of the transmission lines are placed across the transmission lines to terminate the pair. Thus, if the characteristic impedance of a transmission line is 50 ohms, then the termination resistor R is set to 50 ohms. However, there is a problem with this approach in that the impedance of elements at the input interface other than the termination resistor can vary, and therefore affect the value of the terminating impedance. Because of this, signal reflection in such systems contributes to a significant portion of signal loss, especially when signal frequency is in the high radio frequency (RF) range or microwave frequency domain.
Further, in the above approach, electronic circuits can be impedance matched to the transmission line only if the number of electronic circuits coupled to the line and the input impedance of such circuits remains fixed. In a complex system having many components, however, the number of electronic circuits coupled to transmission lines can frequently vary. When the number of circuits change, or the termination impedance of the circuits change, impedance mismatch is likely. By way of an example, suppose that a number of memory modules of a computer system are coupled to a memory controller through a data bus and an address bus, and the terminating impedance of the memory modules is matched to the buses when the computer system is initially placed in service. However, when additional memory modules are coupled later to the buses, because there are more components then, the buses might no longer be terminated in matched impedances.
A different approach to reducing reflections at component interfaces has been to seek targets for terminating a transmission line, given a characteristic impedance ZO, through careful control over the terminating impedance, as designed into high-quality package components such as bonding pads, wires, balls and other chip components. In such an approach, the values of resistance, impedance and capacitance (R, L and C respectively) of each chip and package are carefully controlled for the set of frequencies over which the chip operates. Material compatibility, cost, and process variation, among other factors, have so far not yielded a practical and cost-effective solution.
Consequently, a need exists for a practical solution to dynamically minimize system reflections and impedance mismatch at component interfaces to transmission lines.